This invention is based on a foreign priority application, European Patent Application No. EP 00 307 079.4, filed on Aug. 18, 2000.
The invention relates to a method for manufacturing a multiple walled tube comprising a rolling of a plated metal strip through at least two complete revolutions to form a tube having at least a double wall which has a plated layer on the inside of the tube, said rolling being followed by a heating of the tube to cause the surface of the tube walls, which are in contact with one another, to be brazed.
Such a method is known from FR-1.015.678. According to the known method, a metal strip plated at both sides with copper is used. Once the metal strip is rolled, the tube is heated in order to braze the copper at the contact faces between the walls of the tube. Zinc or tin could be used for the brazing in order to reduce the melting point of the copper.
A drawback of the known method is that the metal strip is plated at both sides with copper. The copper layer at the outer side of the tube has no real technical purpose. During the brazing process, the outer copper layer melts and the melted copper forms droplets on the outer tube wall leading to an unequal surface. Moreover, the outer copper layer reduces the heat transfer inside the tube when heat is applied by means of radiation or induction. The copper layer on the outer wall also imposes some manufacturing constraints such as the use of a black coating during the brazing process. As this black coating renders the brazing device dirty, a regular cleaning is required. When the tube is heated by applying a current to it by direct contact, the melted copper affects the electrical contacts at high temperature.
An object of the present invention is to provide a method for manufacturing a multiple walled tube that is less cumbersome to manufacture without affecting the quality of the manufactured tube.
For this purpose a method according to the invention is characterised in that said metal strip is plated on one side, the other side being formed by the steel of the metal strip and wherein said brazing is realised by brazing directly the plated side on the steel. By using a monoplated metal strip i.e. only plated at one side, the brazing is realised between the steel of the metal plate and the copper. As there is no longer copper on the outer tube wall, the copper can no longer form droplets on the outer side and thus not adversely affect the shape of the tube. The heat transfer towards the inner side of the tube is also improved, as the copper can no longer affect the thermal transfer. As the steel is on the outer side, there is no longer a problem of copper accumulation on the electric contacts during heating, if the latter is realised by means of direct electrical current. The method according to the present invention overcomes the technical prejudice that in order to manufacture a multiple walled tube, a double plated metal strip needs to be used. The skilled person would not even consider to use a monoplated metal strip, since the prior art teaches to use double plated and to solve brazing problems by using an additional layer such as tin or zinc which is superposed or forms an alloy with the copper layer. Surprisingly it has been found that if heat is applied by electromagnetic induction, the copper layer on the outer side acts as an electromagnetic shielding for the steel and restricts considerably the heat transfer to the interface between the walls, where the brazing should be applied. By using a monoplated metal strip, there is no longer a copper layer acting as an electromagnetic shielding. Consequently, the heat transfer is considerably improved. Further it has also been surprisingly observed that Eddy current testing of the tightness of the tube is improved when applied on tubes manufactured according to the present invention. It has indeed been observed that the testing current mainly flows through the copper skin of the tube, to the detrimental of the metal layer. If no such a copper layer is present, the Eddy current is equally distributed over the steel, enabling a reliable testing which leads to less erroneous test results and avoids unnecessary rejection of tubes.
A first preferred embodiment of a method according to the invention is characterised in that said metal strip is plated with copper, said copper being brazed to the steel of the strip. Copper being particularly suitable for brake-line tubes and being an appropriate material to braze.
Preferably, said brazing is realised by passing the formed tube through a radiation furnace.
Preferably said brazing is realised by applying an electric current by means of electrical contacts, contacting the steel surface. As already mentioned, the absence of copper on the outer side enables to avoid accumulation of copper on the electrical contacts.
Preferably said brazing is realised by inducing an electric current into said tube. As no copper is present on the outer wall, the copper no longer acts as an electromagnetic shielding.
The invention also relates to a method for plating a metal strip to be used for manufacturing a multiple walled tube, wherein a steel sheet is immersed in a first electrolytic bath and consequently in a second electrolytic bath, characterised in that the sheet is plated on both sides with a thin layer in the first bath, and plated on only one side in the second bath, the sheet being consequently immersed in a third electrolytic bath wherein the electrode has inverted polarity with respect to the one of the first and second bath. The inverted polarity enables to remove the copper layer applied in the first electrolytic bath on the side concerned, leaving one side with bare steel.